Since measurement using near-infrared light is simple and safe, it is used for non-invasive measurement of the brain functions (see Non-Patent Document 1) and detection of breast cancer (see Non-Patent Document 2). Diffuse optical tomography (DOT) is an attempt to reconstruct an optical characteristic change or the structure inside a living body from observation data, by calculating a diffusion process of near-infrared light, which is extensively scattered inside a living body. Examples of such near-infrared measurement include continuous-light measurement, time-resolved measurement, and frequency-resolved measurement. Continuous-light measurement is characterized in that the amount of information that can be obtained is small, but the cost is low, the measurement is robust to noise because a large amount of light is irradiated, and the sampling frequency is high. On the other hand, time-resolved measurement and frequency-resolved measurement are characterized in that the amount of information that can be obtained is large, but the measurement is significantly affected by noise, and the cost is high.
In many cases, near-infrared light does not penetrate a living body because it is extensively scattered inside the living body. Thus, for example, in measurement of the brain functions, reflection-type measurement is performed in which light-transmitting probes are arranged on the head surface, and light reflected by diffusion is observed using light-receiving probes also arranged on the head surface. In the measurement of the brain functions, simultaneous measurement using a large number of probes is necessary in order to cover the activity region, and, thus, continuous-light measurement is usually used.
Conventionally, probes are arranged at intervals of approximately 3 cm, which is considered to be suitable for measurement of the brain functions, and the observation result is obtained as topographic data (see Non-Patent Document 3). Meanwhile, in recent measurement of the brain functions, there have been many attempts to increase the number of probes per unit area, thereby performing DOT at high density. Accordingly, it becomes possible to discriminate multiple cortical activity regions in the visual area and the somatosensory area (see Non-Patent Documents 4 and 5).